Various systems have conventionally been proposed for the autofocus apparatus. For example, in silver-salt cameras of single-lens reflex type, autofocusing (AF) that uses the phase difference detection method has been utilized in a many models.
In the AF system using the phase difference detection method the outputs from an AF sensor and hence the distance between two images are different according to a status of proper focus, a status of forward-offset focus, and a status of backward-offset focus. Therefore, the lens is driven to obtain proper focus in such a way that the distance between the images becomes equal to a distance at the proper focus. Amount of movement of the lens, namely an amount of movement of the image surface is then calculated from the distance between the two images.
When the brightness of the object has low contrast then in this phase difference detection method light is flashed from an electronic flash device built into the camera. Phase different with respect to the object is detected based on the light reflected from the object. While detecting the phase difference, proper focus can be obtained with only one output from the sensor, and flash can be used only once under the conditions of the low brightness and low contrast of the object.
On the contrary, digital still cameras or digital video cameras use a system which is called an exploratory AF system. In this system, the lens position at which a high-frequency component of a brightness signal obtained by an image pickup device such as a CCD becomes maximum is determined as a focus position.
For example, an autofocus apparatus that uses the exploratory AF system is disclosed in Japanese Patent Laid-Open Publication No. HEI 5-268505. The autofocus apparatus disclosed in this publication comprises a flash for flashing light onto an object, a light receiver for receiving a light reflected from the object, a flash-light quantity changing unit for varying a light quantity of the flash. Further, an AF computing unit is provided for AF computing. Further, a lens is provided for accumulating the light reflected from the object and a lens driving unit is provided for driving the lens. A control section is provided for controlling each of the above-mentioned components. The control section controls in such a way that the AF computing unit does not get saturated.
The conventional technology described above gives good results in a case where AF is slow in a direction of increasing the quantity of the light to be flashed by successively increasing a driving time of the electronic flash device so as to match the change in the distance while successively driving a focal lens to a further position. However, because an AF evaluated value is computed by using a brightness signal this value becomes larger as the brightness signal becomes larger. This fact disadvantageously causes a focus position of the focal lens not to be a peak of a sampled AF evaluated value.